Diffusion coatings resistant to oxidation for γ-TiAl by pack codeposition of Al and Si

Abstract

Pack powder mixtures containing elemental Al and Si powders as the depositing source instead of Al-Si alloy powders (as conventionally used) were used to codeposit Al and Si to form diffusion coatings on γ-TiAl at 1100°C by the pack cementation process in an attempt to increase the high temperature oxidation/corrosion resistance of the alloy. The coatings produced had a coherent two layer structure consisting of an outer silicide layer and an inner TiAl₃ layer. It was suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of Al and Si. The oxidation process of the coating in air over the temperature range up to 850°C was investigated by intermittent weight gain measurement at room temperature. The parabolic rate law of oxidation was applied to analyse the oxidation kinetics of the coatings. X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) techniques were used to identify the oxide scales formed by the oxidation process and to assess the thermal stability of the coatings at the oxidising temperatures. It was revealed that the preferential Si oxidation with the formation of SiO₂ as the scale was the major mechanism controlling the oxidation process. The initial oxidation process involved the conversion of Ti₅Si₄to Ti₅Si₃, leading to a lower parabolic rate constant of oxidation after total conversion. The coatings are thermally stable and can provide effective protection against oxidation for γ-TiAl over the temperature range studied.